A blend (usually referred to as “polyblend”) of an acrylonitrile-butadiene copolymer rubber (NBR) with a vinyl chloride resin (PVC) has heretofore been used as a material for producing a crosslinked rubber product for which an oil resistance and an ozone resistance are required. However, when PVC is incinerated, dioxin which is a pathocrinia substance is produced unless the incineration temperature is carefully and precisely controlled. Therefore incineration of PVC is avoided and the demand for PVC is diminishing.
As a substitute material for the polyblend which has good oil resistance and ozone resistance, a blend of NER with an epthalohydrin rubber is known. NBR and an epihalohydrin rubber exhibit different crosslinking mechanisms and thus the rubber blend is impossible to crosslink with a single crosslinking agent. Attempts for co-crosslinking the two rubbers by using a combination of crosslinking agents which are effective for crosslinking the respective rubbers have been made.
For example, a method of co-crosslinking a blend of a diene elastomer and an epihalohydrin polymer was proposed wherein an organic polysulfide, which is a sulfur-donating compound capable of crosslinking a diene rubber, and 2-mercaptoimidazoline or thiourea, which is a crosslinking agent for an epihalohydrin polymer, and an oxide of magnesium, calcium, zinc or lead are incorporated together in the blend (Japanese Examined Patent Publication No. S50-4032). However, where an oxide of magnesium, calcium or zinc is incorporated, the resulting co-crosslinked product has poor thermal aging resistance and dry physical properties, especially tensile strength, and the rate of crosslinking for the production thereof is low and the productivity is poor. Lead oxide is toxic and its use is restricted. Therefore the above-proposed method of co-crosslinking diene elastomer/epihalohydrin polymer has poor practical use.
An attempt of co-crosslinking a blend of NBR and epihalohydrin rubber was made wherein sulfur as a crosslinking agent and tetramethylthiuram monosulfide or benzothiazolyl disulfide as a crosslinking accelerator are used in combination. But, the crosslinked product has poor ozone resistance.